Interestingly enough, my understanding is the same firm is contracted by both Mercedes and Porsche to design their convertible tops.  I don't think it's the mid-engine design so much as the Boxster's top basically lies a-midship.  As mechanical tops stow in the back, perhaps Bmussatti is right in that the dynamics might be thrown off without a front-mounted engine to counter-balance the weight in the rear.
But then again Porsche obviously has a long tradition of making cars with too much weight to the rear  
 
		
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